Random wavelength meter

ABSTRACT

An optical system comprising a randomizer that has a plurality of randomly positioned scatterers for scattering and thereby randomizing light to generate a speckle pattern and a detector for detecting the speckle pattern to determine at least one property of the light and/or change in at least one property of the light.

FIELD OF THE INVENTION

The present invention relates to an optical system, such as a wavelengthmeter, for example a spectrometer or interferometer, and a method forwavelength selection.

BACKGROUND OF THE INVENTION

Light propagation through time dependent disordered or random media isgenerally regarded as a randomisation process of the optical fielddestroying all the information in the initial beam. However, a coherentbeam propagating in a stationary random medium yields a deterministicspeckle pattern, whilst maintaining its initial spatial and temporalcoherence. Such behaviour is exploited in the design of several noveloptical devices, for example to create focal spots using computergenerated holograms, to trap micro-particles and coherently addressplasmonic nano-structures.

Key to devices based on time dependent disordered or random media isthat the information content of the optical field is maintained whentransmitted through a random medium. Thus, the stationary wavefrontrandomisation process can be used to detect the state of the light fieldbefore scattering.

The use of wavelength meters is ubiquitous in photonics. Miniaturisationof such devices would be highly advantageous. A multimode fiber may beused to create wavefront randomisation to act as a spectrometer, asdescribed in B. Redding, S. M. Popoff, and H. Cao, Opt. Express 21, 6584(2013), and B. Redding and H. Cao, Opt. Lett. 37, 3384 (2012). However,to achieve a resolution of 8 pm between two adjacent laser lines wouldrequire 20 m of fibre free of perturbations, which would be difficult torealize in practice. It has also separately been recognised thatspectral polarimetric measurements may be performed using thetransmission matrix of random media, see T. W. Kohlgraf-Owens and A.Dogariu, Opt. Lett. 35, 2236 (2010).

Lab-on-a-chip applications require small integrated wavelengthdetectors. One way to achieve this is by propagating light throughperiodic structures, such as a super prism made from speciallyengineered photonic crystals. The optical dispersion of these crystalscan deliver resolution of 0.4 nm at a wavelength of 1.5 μm. However,these devices rely on out-of-plane detection and free space propagation,and so are not fully integrated on-chip devices.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an optical systemor apparatus comprising a randomizer that includes randomly positionedparticles for randomizing light to provide a speckle pattern and adetector for detecting and analyzing the randomized light to determineone or more properties of the light. Preferably, the randomizer istransmissive.

The one or more properties of the light may be selected from:wavelength, polarization, coherence and beam shape parameters.

The system or apparatus may be a wavelength meter or a spectrometer oran interferometer.

Preferably, the randomizer comprises a thin layer or film. The thicknessof the thin layer or film may be less than 100 μm, and ideally less than50 μm.

The present invention provides a wavefront mixing process that acts as ageneralised interferometer, delivering a different speckle pattern foreach different incident beam. This property can be used, for example, tosimultaneously measure the azimuthal and radial modes ofLaguerre-Gaussian beams. The same approach can be used to measure otherkey properties of the light field such as polarisation state orwavelength.

The randomizer may comprise a layer of film of randomly positionedparticles, for example aluminium particles, which cause a specklepattern to be formed.

The randomizer may comprise randomly positioned particles suspended in amatrix. The matrix may comprise bulk material or may be a thin planarlayer.

The randomizer may comprise a layer of randomly positioned particles,for example aluminium particles. The randomizer may comprise a layer orslice of biological material, such as a slice of biological tissue. Therandomizer may be provided as a thin film that can be positioned infront of or on an optical path from a light source.

The randomizer may be positioned in an optical cavity, for example aFabry Perot cavity.

The randomizer may be reflective. The randomizer may comprise a hollowelement for internally reflecting and randomizing light to generate aspeckle pattern. The randomizer may comprise a hollow sphere, forexample an integrating sphere, or a hollow tube.

A single mode fibre may be provided for transmitting single mode lightto the randomizer. This avoids issues with beam size matching andincident beam dimensions.

Principal component analysis (PCA) may be used to analyse the randomizedlight to determine the wavelength of the light.

A variable optical element or device may be provided in front of therandomizer for varying the light incident on the randomizer. Thevariable optical element or device may be operable to vary the amplitudeand/or phase of light. The variable optical element or device maycomprise at least one of a deformable mirror, a spatial light modulator,for example a liquid crystal spatial light modulator, and a digitalmicro-mirror.

Multiple randomisers may be provided. The randomisers may beperiodically spaced. The randomisers may be positioned to deliver aspeckle pattern that is most efficient at a specific wavelength.

According to another aspect of the invention, there is provided a lasercomprising a controllable laser source, a randomizer for randomizinglight from the controllable laser source to generate a speckle pattern;a detector for detecting and analyzing the speckle pattern to determineone or more properties of the light; a controller for controlling thecontrollable laser source based on the determined one or more propertiesof the light.

Preferably the randomiser comprises a plurality of randomly positionedscatterers for scattering and thereby randomizing light to generate aspeckle pattern. The randomiser may comprise a thin layer or film ofrandomly positioned particles. The randomiser may comprise a matrix inwhich randomly positioned particles are suspended. The randomiser maycomprise bulk material.

The randomizer may be reflective. The randomizer may comprise a hollowelement for internally reflecting and randomizing light to generate aspeckle pattern. The randomizer may comprise a hollow sphere, forexample an integrating sphere, or a hollow tube.

According to yet another aspect of the invention, there is provided alaser stabilisation system for stabilising the output of a controllablelaser source, the stabilisation system comprising a randomizer forrandomizing light from the controllable laser source to generate aspeckle pattern; a detector for detecting and analyzing the specklepattern to determine one or more properties of the light; and acontroller for controlling the controllable laser source based on thedetermined one or more properties of the light. Preferably therandomiser comprises a plurality of randomly positioned scatterers forscattering and thereby randomizing light to generate a speckle pattern.The randomiser may comprise a thin layer or film of randomly positionedparticles. The randomiser may comprise a matrix in which randomlypositioned particles are suspended. The randomiser may comprise bulkmaterial.

Multiple detectors may be provided and at least part of the specklepattern is incident on the multiple detectors. Different parts of thespeckle pattern may be incident on different detectors. Differentdetectors may be operable to determine different properties of thelight. The different detectors may be operable to simultaneouslydetermine the different properties of the light.

According to yet another aspect of the invention, there is provided anoptical system comprising a randomizer for randomizing light to generatea speckle pattern, at least one detector for detecting and analyzing thespeckle pattern to determine one or more properties of the light, and avariable optical element or device in front of the randomizer forvarying the light incident on the randomizer. The variable opticalelement or device may be operable to vary the amplitude and/or phase oflight. The variable optical element or device may comprise at least oneof a deformable mirror, a spatial light modulator, for example a liquidcrystal spatial light modulator, and a digital micro-mirror. Multipledetectors and means for diverting or directing the speckle pattern tothe multiple detectors may be provided. The means for diverting ordirecting may be operable to divert different parts of the specklepattern to different detectors. The means for diverting or directing maycomprise one or more optical devices or elements. For example, the meansfor diverting or directing may comprise a controllable beam shapingdevice, such as a deformable mirror, spatial light modulator, digitalmicro-mirror.

According to still another aspect of the invention, there is provided anoptical system comprising a randomizer for randomizing light to generatea speckle pattern, multiple detectors for detecting and analyzing thespeckle pattern to determine one or more properties of the light, andmeans for diverting or directing the speckle pattern to the multipledetectors. The means for diverting or directing are operable to divertdifferent parts of the speckle pattern to different detectors. The meansfor diverting or directing may comprise one or more optical devices orelements. For example, the means for diverting or directing may comprisea controllable beam shaping device, such as a deformable mirror, spatiallight modulator, digital micro-mirror. A variable optical element ordevice may be provided in front of the randomizer for varying the lightincident on the randomizer.

Using the scattering properties of a remarkably simple thin diffuser, itis possible to detect the wavelength of a monochromatic beam topicometer precision. This approach may be extended to even higherresolution through the use of an optical cavity placed around therandomizing medium. This allows an ultra-compact spectrometer and newmethods for laser/beam stabilisation based on analysis of the specklefields.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the invention will now be described by way of exampleonly and with reference to the accompanying drawings, of which:

FIG. 1 is a schematic view of a speckle pattern spectrometer;

FIG. 2 (a) shows a PCA decomposition of a detected speckle pattern as afunction of the laser wavelength varying between 785.1 nm and 785.6 nm;

FIG. 2 (b) shows an example of a far-field speckle pattern observed at785.234 nm;

FIGS. 2(c) to (d) show the first three principal components PC₁, PC₂ andPC₃ which correspond to the first three degrees of freedom detectable bythe speckle pattern;

FIG. 3 shows measured wavelength error distribution in the case of thealumina;

FIG. 4 shows modelled speckle pattern variability for a random diffuserinside a Fabry Perot cavity;

FIG. 5 is a schematic diagram of a laser stabilization system that usesspeckle pattern detection to control laser parameters;

FIG. 6 is a schematic diagram of a wavelength spectrometer including anintegrating sphere;

FIG. 7 is a schematic diagram of a tube-based assembly of a wavelengthspectrometer;

FIG. 8 is a schematic diagram of various stages of operation of aspeckle pattern spectrometer;

FIG. 9 is a schematic diagram of another laser stabilization system;

FIG. 10 shows high resolution training and validation of a laser systemusing a first method, and

FIG. 11 shows high resolution training and validation of a laser systemusing a second method.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention uses random scatterers to generate specklepatterns from coherent light, so that properties of the light can bemeasured, such as wavelength, polarization and coherence. Light that haspassed through the random scatterers is analysed using principalcomponent analysis. Before the random scatterers, a coherent beam can beseen as a superposition of many beamlets. After its propagation throughthe random scatterers, an interference pattern is observed between theconstituent beamlets, each having changed directions, spot sizes andrelative phases.

FIG. 1 shows spectrometer that has a tunable laser source that outputsvariable wavelength light into a single mode fibre. Light emitted fromthe fibre is incident on a transmissive randomizer that forms a specklepattern. Light that has passed through the randomizer is incident on aCCD camera (Pike, Allied Vision Technologies, pixel size: 7.4 μm×7.4μm). The speckle pattern detected at the detector is wavelengthdependent, and can be used to determine the wavelength of light from thesource.

Two laser sources were used to test the spectrometer of FIG. 1: atuneable narrow linewidth Littman cavity diode laser system (SacherLasertechnik, 785 nm, line width<1 MHz, TEC-510-0780-100) and aTi:sapphire laser (Spectra-Physics, line width 0.5 GHz, tuneable 700 nm1000 nm, model 3900S). A HighFinesse/Angstrom WS7 super-precisionwavelength meter was used for an independent calibration of each tunablelaser source. The diode laser source was used in the narrow wavelengthrange study (≈0.5 nm), whereas the Ti:sapphire laser source allowedtesting over a larger wavelength range. To exclude laser beamvariability when tuning the laser, both laser beams were filtered bycoupling their outputs into single mode fibers.

Two different geometries were considered for the randomizer. In a firstapproach, a thin layer of random aluminium particles was used. This wasformed using a small drop≈5 μl) of a commercially available solution ofalumina particles with a mean size of 5 μm and deionised water on aglass substrate. The glass slide was 160 μm thick and was cleaned with 5minutes long immersions in Acetone and Isopropanol in an ultrasonicbath, followed by Oxygen-based plasma ashing at 100 W. Care was taken inletting the de-ionised water evaporate slowly to minimise curling of thesurface of the drying drop (see FIG. 1(a-b)). The dried drop wasmeasured to be 40 μm±10 μm thick. In a second approach, two highreflective laser-cavity mirrors within which a random diffuser wasinserted were used to create a randomized Fabry-Perot cavity.

To determine the wavelength corresponding to a given speckle pattern therandom wavelength meter has to be calibrated. This is done by recordingthe speckle pattern for each wavelength to be detected. More precisely,a number N of patterns is measured, where each speckle pattern isdefined by a two dimensional array corresponding to the intensitiesmeasured by the CCD camera. This delivers a higher order arraycorresponding to the intensities measured by the camera A_(ijk) wherethe subscripts i and j are the pixel coordinates on the camera and k anindex distinguishing between different measurements. These differentmeasurements either correspond to different wavelengths λ or to multipleexposures having the same wavelength but probing the fluctuations of theoptical system. FIG. 2(b) shows an example speckle pattern used in thecalibration part of the experiment.

Once calibration is completed, the largest variations between thedifferent speckle patterns are measured using the multivariate principalcomponent analysis (PCA). In a first step, the average speckle image issubtracted from every measured image Â_(ijk)=A_(ijk)−<A>ij where <·>stand for the average over the index k. The pixel coordinates part ofthe intensity array are flattened (for example a 2 by image is flattenedas: pixel (1,1)->1, pixel (1,2)->2, pixel (2,1)->3 and pixel (2,2)->4).This flattening process transforms the higher order array into a normalarray a_(mk)=Â_(ijk) where the index m=1 . . . N corresponds to a uniquemapping from the (i, j) pair to the linear index m. The principalcomponents are obtained by calculating the eigenvectors of the matrix

M=aa^(T)

where the T superscript stands for the matrix transposition. Thecovariance matrix M is N by N sized. Each eigenvector has N elements andcan be recast in the image form by exchanging the linear index m to thepair index (i, j). The eigenvector with the largest eigenvalue is calledthe first principal component (PC), the second largest to the secondprincipal component and so on.

The distribution of eigenvalues allows determination of the number ofdegrees of freedom that the speckle pattern can access as the wavelengthis varied. One method to calculate this number is by determining thenumber of eigenvalues whose sum is equal to 90% or similar threshold ofthe sum of all the eigenvalues. The larger this number is the larger thespeckle pattern variability for a given wavelength variation. Thewavelength resolution of the random spectrometer is higher the largerthe number of degrees of freedom.

The determination of the PC allows the representation of the specklepatterns in PC space. Each measured speckle pattern can de decomposedinto a static background (the average speckle pattern) and the weightedsum of a few principal components accounting most variations. FIG.2(c-d) shows the first three principal components P C₁, P C₂ and P C₃,which correspond to the first three degrees of freedom detectable by thespeckle pattern. The three patterns look similar to each other. However,due to their eigenvector origin these patterns are orthogonal to eachother and each one corresponds to an independent degree of freedom.Indeed, by construction, the matrix M is a positive semi-definitesymmetric matrix whose eigenvectors are orthogonal to each other.

After decomposition, each speckle pattern can be represented by a smallnumber of amplitudes corresponding to the coordinates of a point in thePC space. Here, the first eight PCs were used to represent each pattern.FIG. 2 shows experimental measured wavelength using the aluminarandomizer in direct illumination. FIG. 2 (a) shows a PCA decompositionof the detected speckle pattern as a function of the laser wavelengthvarying between 785.1 nm and 785.6 nm. FIG. 2 (b) shows an example of afar-field speckle pattern observed at 785.234 nm.

FIG. 2a shows the parametric curve described in the PC space (subspacedefined by the first three PC) by the speckle pattern as the wavelengthis varied over a range of 0.5 nm. In this, the parametric curve does nothave a uniform length variation as a function of wavelength. This effectwould have an adverse effect on the uniformity of the wavelengthresolution of our approach. In FIG. 2a , only the first three PC of thedecomposition are represented. FIGS. 2 c) to (e) show the first threeprincipal components used in the decomposition. There are five furtherdecomposition coefficients for each speckle pattern. Taking into accountall eight PC decomposition coefficients greatly diminishes this effectand explains how the PCA method can deliver high resolution.

Once the wave meter is calibrated, the speckle pattern of an unknownwavelength is recorded. This pattern is decomposed in the previouslycalibrated PC space. The wavelength can be established using for examplea nearest neighbour, Mahalanobis distance or linear regressionclassification method. All these classification methods deliver perfectresults (no error) if the detected wavelength is part of the calibrationset.

FIG. 3 shows measured wavelength error distribution in the case of thealumina drop in direct illumination. The bar chart shows the errordistribution (bar chart) for the partial least squares regression and(red curve) for the nearest neighbour classification. The regression hasa standard error deviation of 13 pm. The nearest neighbourclassification was without error. This perfect result can be understoodby considering the approach in the context of wavelength classification.Indeed, for speckle pattern fluctuations smaller than the step size usedfor the speckle pattern training set, the classification approach willalways deliver the nominal classification wavelength with no error atall.

Provided the parametric curve in the PC space is smooth, continuous andlocally linear, it is also possible to measure an unknown wavelengthusing for example partial least squares (PLS) regression in the PCspace. PLS is used to detect the wavelength and determine its standarderror deviation when the unknown wavelength is not necessarily part ofthe calibration set. FIG. 3 shows that the standard deviation of theerror is approximately 13 pm. This can be improved by consideringsmaller wavelength steps in the training set yielding locally smallerdeviations from linear variation between each training step.

There are routes to improve on the sensitivity of the randomizer byincluding an optical feedback mechanism. This can the achieved byembedding the random scattering medium within a Fabry Perot cavity. Themain difference between the two devices was the much lower transmissionintensity through the Fabry Perot based device and the resulting needfor an increased exposure time of the CCD detector. No resolutionimprovement was observed when using the specific Fabry Perot cavity.

FIG. 4 shows modelled speckle pattern variability for a random diffuserinside a Fabry Perot cavity composed of two distributed Brag reflectorshaving an increasing number of periods i.e. increasing reflectivity. Thedifferent colours correspond to different incident wavelengths. FIG. 4shows a potential advantage of this configuration, i.e. the variabilityof the speckle pattern vs. wavelength change is increased as the cavityprovides more feedback. This improvement is ultimately limited by theloss in transmission efficiency.

It is possible to generalise the training method to go beyond thedetection of a single parameter, so that multiple parameters can bemeasured at the same time. This includes not only beam shape parametersbut also polarisation and multiple simultaneous wavelengths. This lattercase enables the construction of compact purpose build spectrometers.Further, the simultaneous detection of changes in multiple beamparameters can give insight into a number of optical phenomena that allhave an effect on the transmission of optical beams. Minute changes inthese parameters can in effect be amplified by the multiple scatteringin the random diffuser and detected with high sensitivity.

Whilst the optical system of FIG. 1 shows a single randomiser, multiplesuch randomisers can be provided. This helps improve sensitivity atspecific wavelengths. The position of each layer is chosen to deliver aninterference pattern that is most efficient at a specific wavelength.The multiple layers are, in effect, periodic in one direction and randomin two directions, so that they behave like a photonic crystal in onedirection and a random scatterer in the other two directions.

The sensitivity, contrast and accuracy of the optical system of thepresent invention can be adjusted through the use of at least onecontrollable device that can control the amplitude or phase of a lightfield. Such controllable devices include deformable mirror is, liquidcrystal spatial light modulators and digital micro-mirror devices. Thecontrollable device is positioned at the input of the randomizer. Usingsuch devices, multiple patterns can be generated from the same beam.This increases the amount of information that can be measuredsimultaneously.

The invention is generic in that it can be used to detect not only thewavelength but, using suitable training, the polarization state and/orshape of the incident beam. Because of this, it is important to limitthe number degrees of freedom that the random spectrometer is trainedfor. Here, a single mode fibre (SMF) has been used to limit the systemto a single variable, the wavelength. In effect, the SMF acts as theinput slit in a monochromator ensuring that at the output of themonochromator only variations in wavelength generate an intensityvariation. However, replacing the SMF by a multimode fibre or pinholewould add to the wavelength variability of the speckle pattern thevariations due to the beam shape.

Using a simple random medium in accordance with the invention canprovide a wavelength meter with picometer resolution, exploiting thelarge number of degrees of freedom associated with the lighttransmission through this disordered medium. Implementations of themeter achieved a 13 pm resolution and a bandwidth of 10 nm at awavelength of 800 nm. The concept can be extended to random media withina cavity. This can enhance its wavelength sensitivity at the expense oftransmitted intensity. This concept may be extended to the developmentof specialised spectrometers and for use for laser stabilization.

FIG. 5 shows a laser stabilization system that uses speckle patterndetection to stabilize laser characteristics. This has acomputer-controlled light source, for example a laser or LED, whichemits an output beam along an optical path and through a transmissiverandomizer that is arranged to generate a speckle pattern. At an outputof the randomiser, a detector is provided for detecting the specklepattern, thereby to determine one or more characteristics of the outputbeam. Data relating to the characteristics determined is fed back to thelaser's computer-control system, which varies one or more operationalparameters of the laser to achieve a desired output. For example, theintra/extra cavity grating could be varied, as could the cavity length,the operating temperature and the pump current. Indeed any controllableoperational parameter could be varied based on feedback from thedetector until the desired output is achieved.

In some circumstances, if data acquisition speed is important, some ofthe principal component analysis can be achieved in the optical domain.To do this, a controllable beam shaping device, such as a deformablemirror, spatial light modulator, digital micro-mirror, is placed at theoutput of the randomizer. The controllable beam shaping device is thenarranged to direct certain parts of the speckle pattern to differentdetectors. The intensity of these new beams will correspond to theprincipal components. The detectors can be single photodetectors, quadphotodetectors, balanced photodetectors or an array of one or more ofthese detectors. Adding or subtracting photocurrents from the multipledetectors, and applying the appropriate weighting factors, provides thePC coefficient for fast data acquisition. This is typically needed forimplementing a feedback loop, as would be required for the laserstabilisation system of FIG. 5.

FIG. 6 shows another example of a randomiser. This has a fibre fordelivering laser light to an integrating sphere and a camera forcapturing a speckle pattern generated by the integrating sphere. Thefibre is a single mode fibre that ensures wavelength change does notaffect beam shape. The integrating sphere is arranged to provide spatialrandomisation of the input light field with low loss.

The integrating sphere has a coating that is highly reflective at thewavelength of interest on its inner surface. The coating diffuses lightin a manner similar to the thin diffuser described above, but inreflection. Light reflects back and forth inside the sphere untilultimately it is detected at the output port with the camera or thearray of detectors. Inside the sphere, a number of coated baffles (notshown) is provided to block a direct light path between input and outputport. The material of the sphere ideally should have high thermalstability, so as not to change its optical properties as a result ofsmall temperature fluctuations. The sphere may also be stabilised intemperature using thermoelectric Peltier elements, for example. Furthertemperature stabilisation can also be achieved by cooling theintegrating sphere in a constant temperature liquid bath. The surfacesinside the integrating sphere are treated to ensure high diffusivereflection. The imaging part of the integrating sphere corresponds to anoutput port of the integrating sphere. No further optics are required onthis port, although one or more optical elements could be used toenlarge or shrink the speckle pattern.

The laser beam that is to be measured or stabilized is optically coupledto the single mode fibre. Light coming out of the single mode fibre doesnot change its beam profile as the wavelength changes. The output lightfrom the fibre is used to illuminate the input port of the integratingsphere. The integrating sphere creates highly wavelength sensitivespeckle patterns. This is achieved through the many diffuse reflectionsinside the sphere that the light field makes before reaching the outputport and the camera. In effect, the camera measures the speckle patterncreated from the interference between the many paths the light is takinginside the sphere. As the distances between successive diffusivereflections inside the sphere are large this speckle pattern has a highsensitivity to small wavelength changes. In general, the specklewavelength resolution/sensitivity is proportional to the opticalpath-length inside the speckle-generating device.

FIG. 7 shows another example of a hollow, reflective randomiser. In thiscase, the randomiser is a reflecting tube with multiple paralleldiffusers (for example as described previously) inside it to create alow loss high stability optical arrangement. FIG. 7 shows two diffusers,but it will be appreciated that more could be used to form a cascadedseries. Optionally, an inner surface of the tube may be coated with ahighly reflective, diffusive material, as described with reference tothe integrating sphere. As before, a fibre is used to deliver laserlight to the reflective randomiser and a camera is provided forcapturing a speckle pattern generated by the reflective randomiser. Thefibre is a single mode fibre that ensures wavelength change does notaffect beam shape. The tube randomiser is arranged to provide spatialrandomisation of the input light field with low loss and high stability.

In the examples shown in FIGS. 6 and 7, the detector used is a camera.However, any suitable detector could be used, for example an array ofdetectors, such as an array of quadrant photo diode (QPD) detectors.Also, it should be noted that the shape and size of the hollowreflector, for example the sphere or tube, can be changed or engineeredto achieve different wavelength sensitivities and environmentalstability (temperature, etc).

FIG. 8 shows a method for measuring average wavelength changes of alaser beam whose wavelength is modulated periodically. The first halfperiod is used to train (calibrate) the speckle pattern using theprincipal components method and then subsequent half periods are used tovalidate (double check) the training. In this instance, the calibrationand the validation steps need an external wavelength meter to deliverabsolute wavelength measure that can be used to calibrate the specklepattern. In the validation step the external wavelength meter is used toverify the wavelength detected by the speckle wavelength meter. The lefthand figure shows the wavelength changes as a function of time asmeasured by a standard wavelength meter superimposed by the wavelengthdetected by the speckle spectrometer. The right hand figure shows theschematics of the setup consisting of the laser beam, randomiser, cameraand detected speckle pattern. The lower part of FIG. 8 illustrates theprincipal components training approach. The four panels correspond tothe projections of the detected speckle patterns onto the principalcomponents. In order, these are: the first principal component versusthe second principal component; the first principal component versus thethird principal component; the second principal component versus thethird principal component and the second principal component versus theforth principal component.

The method described with reference to FIG. 8 requires the use of anexternal wavelength meter for calibration. In another approach, the needfor an external meter can be avoided. In this case, one or more opticalcomponents are varied in such a way as to cause periodic oscillations inthe laser output, for example the wavelength. These periodicoscillations can be detected as periodic oscillations in the first fewprincipal components. Knowing the oscillation amplitude enables themeasurement and detection of relative wavelength changes that can beused to create a feedback loop without prior calibration. This approachcan be used to minimise the effect of thermal drift by monitoring highfrequency periodic wavelength oscillations. If the frequency of thewavelength modulation is higher than the bandwidth of the thermalfluctuations then it is possible to measure wavelength drifts of thelaser beam in a time short enough to avoid any interference with thethermal drift of the speckle pattern generating device.

FIG. 9 shows a laser with a wavelength stabilisation system inaccordance with the invention. This has a laser cavity defined by tworeflectors, one of which allows some light to be output. In the cavity,there is provided a gain medium and an intracavity element. At an outputof the laser is a speckle pattern wave meter. In the example shown, thespeckle pattern wave meter has an integrating sphere and acamera/detector that is connected to a controller. However, any of theother speckle pattern wave meters described herein could be used. Thecontroller is connected to the first mirror, the gain medium and theintracavity element. Each of these three components can be controlled bythe controller to vary the wavelength at the laser output. Hence, thelaser output can be influenced via multiple channels. By varying theposition of the mirror, the cavity length can be changed. By varying thepotential applied to the gain material the laser gain can be varied. Byvarying the intracavity active elements (such as filters or beamshapers), for example by moving them into or out of the optical path,again the laser output can be varied. Each of these laser controlchannels can be used to create changes in laser beam properties, whichcan be detected as changing speckle pattern after the randomiser.

The system of FIG. 9 can be calibrated to compensate for relativewavelength changes without using an external wave meter. As an example,the input channels controlling the laser could be modulated at high(known) frequency (above thermal noise). Each channel can be modulatedwith a different frequency. Then variations of the speckle pattern ateach modulation frequency are determined. These variations can bedetermined using real-time multivariate analysis such as principalcomponents analysis or singular value decomposition. Monitoring theseoscillations allows any drift in the laser system to be detected. Thesecan then be counteracted via the laser control channels.

FIG. 10 and FIG. 11 show the results from the two approaches. FIG. 10shows high resolution training and validation using an externalwavelength meter resolving 10 pm. In FIG. 10, the left hand side is thetraining data and the right hand side is the validation. FIG. 11 showshigh resolution training and validation using the second approach, i.e.the approach using relative properties and avoiding the need for anexternal wavelength meter for calibration. In FIG. 11, the training datais shown in the upper plot. The training is continuous using singularvalue decomposition. This means that the training slowly forgets oldspeckle patterns as new speckle patterns are acquired. The validationdata is shown in the lower plot of FIG. 11. Here the resolution is 20MHz (˜0.1 pm) limited by the laser modulation amplitude.

In lasers, the optical properties of the active gain medium inside thelaser cavity are highly temperature dependent. Usually, the gain mediumis temperature stabilised through the use of a thermostat which measuresthe temperature of the gain medium using, for example, a thermo couple.By monitoring speckle pattern variations in accordance with the presentinvention, the impact on the output due to changes in the temperature ofthe gain material can be monitored and a feedback loop used to stabiliseits optical properties directly (for example by varying the drivecurrent).

As well as being used to stabilise lasers, the present invention couldbe used to stabilise other optical components, such as optical sensors.In particular, the speckle patterns of the invention can be used tostabilise, control or monitor optical components, which have outputsthat are temperature dependent. The temperature dependence could be dueto thermal expansion, contraction and refractive index changes withtemperature. In practice, some of these changes are minute. However,speckle pattern changes can be used to detect these minute changes andmeasure an effect temperature change in the optical system. Thistemperature change can then either be monitored or used in a feedbackloop adapted to control one or more parameters that effect temperature.The temperature dependent optical changes might not be associated withwavelength change only, but beam shape and polarisation might alsochange. In this case the speckle pattern device would not include thesingle mode fibre at the input which is used when detecting onlywavelength changes.

In all the examples described above, the detector may include aprocessor or analyser for analyzing the speckle patterns to determineone or more parameters of the light and/or changes in such parameters.Alternatively, the analysis processor or analyser may be providedseparately from any detector element. Equally, in all cases, multipledetectors or arrays of detectors may be provided and at least part ofthe speckle pattern may be incident on the multiple detectors. Differentparts of the speckle pattern may be incident on different detectors.Different detectors may be operable to determine different properties ofthe light. The different detectors may be operable to simultaneouslydetermine the different properties of the light.

The present invention provides a high resolution, high sensitivityspeckled pattern wavelength meter. This allows the locking of a laserwavelength at any chosen wavelength. Additionally, it enables anultrastable light source in the continuous wave and pulsed operationalregimes of a laser device, because it is possible to counteracttemporal, spectral, spatially and amplitude fluctuations of the laserdevice. Also, the characterised speckled pattern can be used as a “dialon demand” speckle pattern for structured illumination for imagingapplications.

A skilled person will appreciate that variations of the disclosedarrangements are possible without departing from the invention. Forexample, whilst the analysis technique has been described based on PCA,it will be appreciated that other pattern detection methods could beused. Also, whilst the specific embodiment uses a single mode fibre tofilter the randomised input beam, this is not essential. As well asbeing sensitive to wavelength, the speckle pattern is sensitive to beamshape and polarisation of the light field. Where information on theseparameters is needed, the single mode fibre would not be used.Accordingly, the above description of the specific embodiment is made byway of example only and not for the purposes of limitation. It will beclear to the skilled person that minor modifications may be made withoutsignificant changes to the operation described.

1. An optical system comprising a randomizer that has a plurality ofrandomly positioned scatterers for scattering and thereby randomizinglight to generate a speckle pattern and a detector for detecting thespeckle pattern to determine at least one property of the light and/orchange in at least one property of the light.
 2. An optical system asclaimed in claim 1 wherein the one or more properties comprise one ofmore of wavelength of the light; polarization of the light and spatialmode
 3. An optical system as claimed in claim 1 wherein the randomizercomprises randomly positioned particles, for example aluminiumparticles.
 4. An optical system as claimed in claim 1 wherein randomizercomprises randomly positioned particles in a thin film or layer.
 5. Anoptical system as claimed in claim 1 wherein randomizer comprisesrandomly positioned particles suspended in a matrix.
 6. An opticalsystem as claimed in claim 1 wherein the randomizer comprises biologicalmaterial that includes randomly positioned scatterers.
 7. An opticalsystem as claimed in claim 6 wherein the biological material comprisesbiological tissue.
 8. An optical system as claimed in claim 1 whereinthe randomizer is in an optical cavity.
 9. An optical system as claimedin claim 8 wherein the optical cavity is a Fabry Perot cavity.
 10. Anoptical system as claimed in claim 1 comprising a single mode fibre fortransmitting single mode light to the randomizer.
 11. An optical systemas claimed in claim 1 wherein the detector is arranged to use principalcomponent analysis (PCA) to analyse the randomized light to determinethe wavelength of the light.
 12. An optical system as claimed in claim 1wherein the randomizer comprises a layer of film less than 100 μm thick,and preferably less than 50 μm thick.
 13. An optical system as claimedin claim 1 wherein the randomizer is transmissive.
 14. An optical systemas claimed in claim 1 wherein the randomizer is reflective.
 15. Anoptical system as claimed in claim 14, wherein the randomizer comprisesa hollow element for internally reflecting and randomizing light togenerate a speckle pattern.
 16. An optical system as claimed in claim 15wherein the randomizer comprises a hollow sphere, for example anintegrating sphere, or a hollow tube.
 17. An optical system as claimedin claim 1 further comprising a variable optical element or device infront of the randomizer for varying the light incident on therandomizer.
 18. An optical system as claimed in claim 17 wherein thevariable optical element or device is operable to vary the amplitudeand/or phase of light.
 19. An optical system as claimed in claim 17wherein the variable optical element or device comprises at least one ofa deformable mirror, a spatial light modulator, for example a liquidcrystal spatial light modulator and a digital micro-mirror.
 20. Anoptical system as claimed in claim 1 comprising multiple detectors andmeans for diverting the speckle pattern to the multiple detectors. 21.An optical system as claimed in claim 20 wherein the means for divertingare operable to divert different parts of the speckle pattern todifferent detectors.
 22. An optical system as claimed in claim 20wherein the means for diverting comprise a controllable beam shapingdevice, such as a deformable mirror, spatial light modulator, digitalmicro-mirror.
 23. An optical system as claimed in claim 1 arranged as awavelength meter or a spectrometer or interferometer.
 24. An opticalsystem as claimed in claim 1 comprising multiple randomisers.
 25. Anoptical system as claimed in claim 1 comprising a laser, the systembeing adapted to monitor changes in the laser output by monitoringspeckle patterns as a function of time.
 26. A laser comprising acontrollable laser source, a randomizer for randomizing light from thecontrollable laser source to generate a speckle pattern; a detector fordetecting the speckle pattern to determine one or more properties of thelight and/or changes in one or more properties of the light; and acontroller for controlling the controllable laser source based on thedetermined one or more properties of the light and/or change in one ormore properties of the light.
 27. A laser as claimed in claim 26 whereinthe randomiser comprises a plurality of randomly positioned scatterersfor scattering and thereby randomizing light to generate a specklepattern.
 28. A laser as claimed in claim 27 wherein the randomizer istransmissive.
 29. A laser as claimed in 26 wherein the randomizer isreflective.
 30. A laser as claimed in claim 29, wherein the randomizercomprises a hollow element for internally reflecting and randomizinglight to generate a speckle pattern.
 31. A laser as claimed in claim 30wherein the randomizer comprises a hollow sphere, for example anintegrating sphere, or a hollow tube.
 32. A laser as claimed in claim26, wherein the controller is operable to control or vary at least oneof a length of a laser cavity and a laser gain medium.
 33. A laser asclaimed in claim 26, wherein the controller is operable to control orvary at least one property of an intracavity element.
 34. A laserstabilisation system for stabilising an output of a controllable lightor laser source, the stabilisation system comprising a randomizer forrandomizing light from the controllable laser/light source to generate aspeckle pattern; a detector for detecting the speckle pattern todetermine one or more properties of the light and/or changes in one ormore properties of the light; and a controller for controlling thecontrollable laser source based on the determined one or more propertiesof the light and/or changes in one or more properties of the light. 35.A system as claimed in claim 34 wherein multiple detectors are providedand at least part of the speckle pattern is incident on the multipledetectors.
 36. A system as claimed in claim 35 wherein different partsof the speckle pattern are incident on different detectors.
 37. A systemas claimed in claim 35 wherein the different detectors are operable todetermine different properties of the light.
 38. A system as claimed inclaim 37 wherein the different detectors are operable to simultaneouslydetermine the different properties of the light.
 39. An optical systemcomprising a randomizer for randomizing light to generate a specklepattern, at least one detector for detecting the speckle pattern todetermine one or more properties of the light, and a variable opticalelement or device in front of the randomizer for varying the lightincident on the randomizer.
 40. An optical system as claimed in claim 39wherein the variable optical element or device is operable to vary theamplitude and/or phase of light.
 41. An optical system as claimed inclaim 39 wherein the variable optical element or device comprises atleast one of a deformable mirror, a spatial light modulator, for examplea liquid crystal spatial light modulator and a digital micro-mirror. 42.An optical system as claimed in claim 39 comprising multiple detectorsand means for diverting the speckle pattern to the multiple detectors.43. An optical system as claimed in claim 42 wherein the means fordiverting are operable to divert different parts of the speckle patternto different detectors.
 44. An optical system as claimed in claim 42wherein the means for diverting comprise a controllable beam shapingdevice, such as a deformable mirror, spatial light modulator, digitalmicro-mirror.
 45. An optical system comprising a randomizer forrandomizing light to generate a speckle pattern, multiple detectors fordetecting the speckle pattern to determine one or more properties of thelight, and means for diverting the speckle pattern to the multipledetectors.
 46. An optical system as claimed in claim 45 wherein themeans for diverting are operable to divert different parts of thespeckle pattern to different detectors.
 47. An optical system as claimedin claim 45 wherein the means for diverting comprise a controllable beamshaping device, such as a deformable mirror, spatial light modulator,digital micro-mirror.
 48. An optical system as claimed in claim 45comprising a variable optical element or device in front of therandomizer for varying the light incident on the randomizer.